1. Field of the Invention
The present invention relates to a magnetoresistance effect device and a method of manufacturing the device, which are applied to a magnetic random access memory (MRAM) and the like.
2. Description of the Related Art
Attention has recently been attracted to a magnetic random access memory (referred to as MRAM hereinafter), which makes use of magnetoresistance effects of a ferromagnet, as a next-generation solid-state nonvolatile memory capable of reading/writing data at high speed, having a large capacity and performing a low power operation. In particular, a magnetoresistance effect device with a ferromagnetic tunnel junction has been noted more and more since it was found that the device greatly varied in magnetoresistance.
The ferromagnetic tunnel junction has a laminated structure of three layers as a basic structure. The three layers are a free layer (magnetized free layer) whose magnetization direction easily varies with an external magnetic field, a pin layer (magnetized fixing layer) which is opposed to the free layer and maintained in a given magnetization direction even though an external magnetic field is applied, and a tunnel barrier layer (insulator layer) which is interposed between the free layer and the pin layer. The pin layer is formed by stacking an antiferromagnetic layer on a ferromagnetic layer and its magnetization direction is fixed. The free layer is generally formed of soft magnetic material whose coercive force is small. In the ferromagnetic tunnel junction, current flows through the tunnel barrier layer used as a tunnel. The resistance of the junction varies with a relative angle between the pin layer and the free layer in the magnetization direction. The resistance is minimized when the magnetization directions of the pin and free layers are parallel and maximized when they are not parallel. This variation in resistance is called a tunneling magneto-resistance effect (referred to as TMR effect hereinafter). When a magnetic device having a ferromagnetic tunnel junction is actually used as one storage cell of an MRAM, it stores information by making the parallel and not parallel states (minimum and maximum of resistance) between the pin layer and the free layer correspond to “0” and “1”.
The write of magnetic information is performed by arranging a write interconnection close to the storage cell and inverting only the magnetization direction of the free layer by a current magnetic field generated when current flows through the storage cell. The read of magnetic information is performed by detecting a variation in resistance due to the TMR effect when sense current flows through the storage cell. The above-described magnetoresistance effect device using the TMR effect is called a magnetic tunnel junction (MTJ) device.
In order to put an MRAM having an MTJ device to practical use as a device, an antiferromagnetic layer, a pin layer and the like are subjected to crystal growth and need to have heat resistance so that the output characteristics (variations of resistance of tunnel junction: MR ratio) do not deteriorate even after heat treatment at about 350° C. that is required in a semiconductor process.
If, however, a generally-used antiferromagnet is heat-treated at about 350° C. in terms of heat stability in a so-called spin bulb type MTJ device, the material of an antiferromagnetic layer diffuses to the interface between a pin layer and a tunnel barrier layer provided on the antiferromagnetic layer to decrease the spin polarizability of a ferromagnet used for the pin layer. Thus, the output characteristics (MR ratio) deteriorate.
Even though the same heat treatment is performed in an MTJ device having a pin layer of a synthetic pin type, the material of an antiferromagnetic layer diffuses to destroy the interface between a ferromagnetic layer and a nonmagnetic layer in the pin layer. Antiferromagnetic coupling is weakened and the pin layer is made unstable in its magnetization direction. Accordingly, the output characteristics (MR ratio) deteriorate.
The material of an antiferromagnetic layer diffuses not only in an interface but also along a crystal grain boundary. In general, the diffusion along the crystal grain boundary is caused more easily than that in the interface (see, for example, FIG. 7 in Jpn. Pat. Appln. KOKAI Publication No. 2004-47583).